Method for purifying waste water using activated slude to increase purification yields

ABSTRACT

The invention concerns a process for increasing the purification yield in the activation tank of a station for the purification of waste waters employing the so-called &#34;activated sludge&#34; process. According to the invention, talc, pyrophyllite, kaolin or mica, possibly cationized, are added to the biomass, these having a particle size less than 100 μm in quantities of up to 2.0 g per liter of waste waters. A great improvement in solid/liquid separation is observed at the outlet from the activation tank as well as an increase in the purification yield for carbonaceous pollution, nitrogenous pollution and phosphate-containing pollution even when the station operates at medium or high load.

The invention concerns a process for the purification of waste waters bybiological means, of the so-called "activated sludge" type.

During the biological purification of waste waters by the activatedsludge process, a process used in more than 60% of public purificationstations in the developed world, waste waters that are often previouslyfreed from sand and degreased are led (sometimes via a pre-separator)into the activation tank where biological degradation is carried out bybacteria. This activation tank is aerated at least during certain cyclesin order to enable aerobic bacteria to develop, essentially with the aimof removing carbonaceous impurities. This tank can also be subjected tonon-aerated cycles for the development of anaerobic bacteria also withthe aim of degrading nitrates and it is also possible to provide asecond non-aerated tank for the development of anaerobic bacteria. Thebacteria or micro-organisms form flocs with a density close to that ofwater (called "activated sludges") and the purified waste waters areseparated from these flocs by density difference, conventionally bygravity in a post-separator (generally called a "clarifier" or"secondary separator" which will from now on be called a "separator").The purified water can then be directly discharged to the river.

Sludges collected at the base of the separator are partially led againto the activation tank so as to maintain a large number of purifyingmicro-organisms. Excess sludges are withdrawn and then treatedseparately so as to reduce their volume and mass, with a view toagricultural use or to be discharged as a sediment.

Operators of stations for the biological purification of waste waters byactivated sludges are confronted with two main difficulties:

1) Maintenance of a sufficient concentration of purifyingmicro-organisms in the activation tank in contact with the water to betreated.

If, by the physico-chemical composition of the waters entering thestation, the bacteria which develop in the medium associate infilamentous flocs which settle very little (40% of stations in Europe),the separation of water/purifying micro-organisms is very difficult toachieve. The separator can no longer play its part in the clarificationof the treated water and the supernatant which is directed to the rivercontains many purifying micro-organisms. The activation tank is thenregularly washed and gradually becomes empty of purifyingmicro-organisms, thus leading to a halt in the purification of usedwaters.

2) Nitrification of the ammonium ions of the effluent to be treatedsince, on the one hand, ammonium ions which constitute the greater partof the nitrogenous pollution are very toxic, and on the other hand,nitrification of ammonia is the limiting stage in the removal ofnitrogenous nutrients responsible in part for the problems ofeutrophication of large and small rivers and lakes.

When the quantities of waste waters to be purified are greater than thetreatment capacity of the station (so-called "medium or heavy load"stations, signifying that the pollution to be treated is very large inrelation to the quantity of purifying micro-organisms present in theactivation tank), the operator can no longer recirculate the sludgescoming from the separator tank through a lack of volume capacity in theinstallation. It is for example the case in stations purifying wastewaters from towns with an increasing population. Activated sludges arethen almost totally removed from the purifying system. It follows thatthe quantity of micro-organisms degrading the carbonaceous pollution inthe activating tank is very low and that there is a total absence ofmicro-organisms degrading the nitrogenous pollution, the latter notbeing present in "young" sludges, that is to say in sludges which havenot been held for a very long time in the station. The carbonaceouspollution entering the station is thus only slightly degraded while thenitrogenous pollution is not degraded at all.

Few solutions exist for maintaining a sufficient concentration ofpurifying micro-organisms (often referred to as a "biomass") in theactivation tank, and/or for introducing nitrogen-fixing bacteria whichare indispensable for the elimination of nitrogenous pollution(particularly NH₄ ⁺ and/or NO₃ ⁻). In the case of a station in under-capacity (quantities of water to be treated greater than the treatmentcapacity), the only solution consists of increasing the size ofactivation tanks (or changing the process). This extension to thestation is very costly and sometimes impossible when the station issituated in an urban area where there is a lack of available groundspace.

Moreover, low concentrations of purifying micro-organisms lead to a verylow yield from the final phases in the removal of nitrates and make itnecessary to increase considerably the size of the installations or toprovide auxiliary tanks for removing this type of pollution.

The object of the present invention is to provide a process whichenables, on the one hand, an increase in concentration of the purifyingbiomass to be obtained in the activation tank and a better control ofthis concentration to be achieved, and on the other hand, an increase inthe activity of the bacteria present to achieved, even when theactivated sludges are present in the form of filaments difficult toseparate from the treated water, and even when the amount of sludgepurged from the separator is high on account of the undercapacity of thestation to treat incoming effluents. This increase in the concentrationand activity of the active biomass first of all concerns micro-organismswhich degrade carbonaceous pollution. Due to the increase in theresidence time of micro-organisms in the activation tank, it alsoconcerns micro-organisms which oxidise and degrade ammoniacal pollution.The improved process aimed at by the invention thus enables a reductionto be achieved in the discharge of organic material into the activationtank, it also concerns micro-organisms which oxidise and degradeammoniacal pollution. The improved process aimed at by the inventionthus enables a reduction to be achieved in the discharge of organicmaterial into rivers as well as removal of toxic ammonium ions for thesame degree of civil engineering construction. Moreover, the increase inthe activity of bacteria results in an increased consumption ofphosphates and hence in an increased degree of removal of phosphatecontaining pollution.

For this purpose, the process of the invention is of the so-calledactivated sludge type, wherein waste waters containing carbonaceouspollution and/or nitrogenous pollution and/or phoshate-containingpollution in an activation tank are put into contact with a biomasscomposed of purifying micro-organisms, the treated water is separatedfrom activated sludge in a separator, and a fraction of the activatedsludge is recycled to the activation tank. According to the presentinvention, a talc or pyrophyllite or mica powder, with the exclusion ofany organic binder, is mixed with the biomass, under conditions suchthat the mineral grains and bacteria are put directly into contact, inorder to encourage the formation of composite flocs consisting ofmineral grains of talc, pyrophyllite, or mica enclosed in a bacterialmatrix and directly attached to this, the activated sludges formed bythe said mixed flocs of mineral grains/bacteria having a density greaterthan that of water and being partially recycled continuously so as toincrease the concentration of micro-organisms in the activation tank andthe residence time of the said micro-organisms in the said tank.

The term "separator" is intended to mean any static or dynamic system,encouraging solid/liquid separation by means of density differences,such as a gravity settling tank, hydrocyclones etc.

In the process of the invention, it has been possible to observe thatwhen the grains of talc, pyrophyllite or mica powder come into contactwith micro-organisms, mixed flocs form by a trapping effect of thepowder grains in the bacterial flocs. This effect is unexpected byreason of the high physico-chemical inertia characteristics,non-porosity and small specific area of the abovementioned powders. Theeffect obtained is all the more unexpected in that talc, pyrophylliteand mica have a well known lipophilic character which gives them anadsorbing power for greases and would lead one to expect a completeabsence of affinity for bacteria (such an affinity generally beingassociated with materials of a hydrophilic nature). The above-mentionedeffect of the formation of mixed flocs is very marked and up to now ithas not been possible to explain it. When the quantity of talc,pyrophyllite or mica powder is between 0.01 g/l and 2.0 g/l of wastewaters, it is noted that practically all the grains of powder and allthe micro-organisms are assembled in a multitude of mixed flocs of theabovementioned type.

The size of talc, pyrophyllite or mica particles used is advantageouslyless than 100 μm. In a particularly advantageous embodiment of theprocess according to the invention, the size of the talc, pyrophylliteor mica particles used is between 3 and 50 μm.

"Talc" is intended to mean hydrated magnesium silicate of the formula3MgO.4SiO₂.H₂ O or any mixture of phyllosilicate containing thiscompound. "Pyrophyllite" is intended to mean hydrated aluminium silicateof the formula Al₂ O₃.4SiO₂.H₂ O, sometimes also known as "Agalmatolite"or any silicate mixture containing this compound. "Mica" is intended tomean aluminium micas such as muscovites of the formula 6SiO₂.3Al₂ O₃.K₂O.2H₂ O, magnesium micas such as phlogopites of the formula 6SiO₂.Al₂O₃,6MgO.K₂ O.2H₂ O, as well as aluminium or magnesium illites derivedfrom the preceding minerals by variable substitutions of Al for Si onthe one hand and Fe for Al or Mg on the other hand. In all cases, thesenatural species may contain associated minerals.

The mixed flocs formed by the process according to the invention have ahigher density than that of simple bacterial flocs by reason of thedensity of the powder grains, of the order of 2.5 to 3 times greaterthan those of aggregates of micro-organisms which usually form in anaqueous medium. This high density enables rapid densification ofactivated sludges to be obtained during successive recycling, whichallows good separation of these to take place in the separator in apermanent operational phase. It should be noted that separation isgreatly encouraged when the separator is a hydrocyclone. The techniqueof hydrocycloning which has the advantage of resulting in compactdevices suitable for the possible treatment of odours, is currently onlyused to a limited extent since the differences in density betweenbacterial flocs and water are often very small and, on account of this,cut-off thresholds are very difficult to maintain. In all cases,recirculation of purifying micro-organisms from the separator to theactivation tank is very largely encouraged, even if the bacteria are inthe form of filaments (which usually come together in bacterial flocsthat settle with great difficulty). Moreover, this improvedrecirculation of activated sludges increases the residence time ofpurifying micro-organisms, enabling nitrifying bacteria to becomeestablished in a surprising way, even in so-called "medium or high load"stations. It has also been noted that, for an equal weight ofmicro-organisms, increased bacterial activity occurs when these areagglomerated into mixed micro-organism/powder grain flocs (compared withthe activity of purely bacterial flocs), this effect also beingunexplained. This increase in activity (measured by oxygen consumptionin the medium) results in an increased consumption of phosphate. Itshould be noted that the aforementioned powders are chemically neutraland are not soluble in waste waters when they are added to these (whichavoids loading waters with new pollution).

A variant of the process according to the invention consists of usingcationized talc, pyrophyllite or mica powders. It involves talc,pyrophyllite or mica powders treated with various cationic agents. As acationic agent, compounds can be used for example such as anines basedon epichlorhydrin, quaternary aliphatic polyamines etc. The use ofcationized talc, pyrophyllite or mica enables densification of activatedsludges to be obtained, exceeding those reached with the samenon-treated mineral powders.

Powders acting as additives can also be introduced, according to theprocess of the invention, both before the pre-separation tank as well asdirectly into the activation tank. It is also possible to introducetalc, pyrophyllite or mica into the recirculation circuit for activatedsludges between the separator and the activation tank or in theseparator (whether conventionally or not, dynamically or statically)where it will act directly on the densification of sludges. It has beendemonstrated that addition of talc, pyrophyllite, or mica powderdirectly to the activation tank is particularly effective for rapidlyincreasing the concentration of micro-organisms. Addition of thesepowders to the separator is particularly effective in the case offilamentous bacteria so as to avoid washing the latter.

The following examples are intended to illustrate the process of theinvention. All these examples are carried out with waste waters drawnoff at the outlet of a primary separator of a conventional typereceiving a hydraulic charge corresponding to an ascending rate of theorder of 0.5 m/h, with the exception of examples 8 and 9.

Examples 1 to 7 were carried out in a pilot station comprising(downstream from this primary separator):

a cylindrical activation tank with a volume of 3.51, supplied at a rateof 1.51/h and continuously oxygenated and agitated (adjustment of theconcentration of oxygen in the medium to 5 mg/l),

a cylindro-conical separator with a volume of 31, the conical base ofwhich is connected, on the one hand, by a pipe for recycling sludges tothe activation tank, and on the other hand connected to an extractionpump,

a system for introducing mineral powder into the activation tank (or forexamples 4, 5 and 6 into the separator), comprising a reservoir ofpowder in suspension in water and a pump for injecting this suspension,enabling the quantity of suspension to be adjusted and thus the quantityof powder introduced into the activation tank (or into the separator).

Examples 1, 2 and 3 on the one hand, and examples 4, 5 and 6 on theother hand, were carried out simultaneously on three installationsidentical to that described previously and referred to as the "pilotinstallation". Example 7 was carried out in this pilot installation, thesingle figure of drawings illustrating the results obtained in thisexample 7.

EXAMPLE 1

Stabilization of a purification station by activated sludges operatingconventionally, by introducing talc powder.

This example was carried out in three successive phases:

First phase

Duration: 35 days of conventional operation in order to define thepurifying properties of the installation.

Second phase:

Duration: 35 days of operation with 0.15 g of talc/litre of waste watersto be treated, introduced continuously into the activation tank. Thisphase enabled the effect of talc on the purifying properties of theinstallation to be demonstrated.

Third phase:

Duration: 45 days of conventional operation, introduction of talc beingdiscontinued at the start of this phase. This phase ensured that theeffect noted in phase 2 was indeed due to the addition of talc and notderived from the operation of the installation, and that in the absenceof talc, the installation purified to the same level as during phase 1.This phase was longer than the preceding ones, since about a week wasneeded to remove the talc completely from the installation.

Three parameters were measured each day during the three successivephases, and their means and standard deviations measured for each phaseof the study

the chemical oxygen demand for all the effluent (total COD, expressed inmg O₂ /1), characteristic of the concentration of carbonaceouspollution, measured at the inlet of the activation tank and at theoutlet from the separator (the reduction in total COD was thuscalculated, the first characteristic of the purifying performances ofthe installation),

the chemical oxygen demand for the soluble fraction of the effluent(soluble COD, expressed in mg O₂ /1), characteristic of theconcentration of soluble carbonaceous pollution, measured at the inletof the activation tank and at the outlet from the separator (thereduction in soluble COD was thus calculated, the second characteristicof the purifying performance of the installation),

the Mohlmann index, expressed by the volume occupied by the activatedsludges allowed to settle for 30 minutes, in relation to the mass ofsuspended matter of the said activated sludges (MI in ml/g) (the smallerthis index, the easier the separation of bacterial flocs/purifiedwater).

The talc powder used was sold by TALC DE LUZENAC (France), under thereference "LUZENAC MB 30". It consisted of 55% hydrated magnesiumsilicate and 45% hydrated magnesium aluminium silicate. 75% of particleshad an equivalent spherical diameter less than 11 μm, 50% less than 6.3μm and 25% less than 3.5 μm.

    ______________________________________                                        Total COD       Soluble COD                                                   (mg/l)          (mg/l)         M.I.                                           V.sub.s  E.sub.t A.sub.b                                                                              V.sub.s                                                                             E.sub.t                                                                            A.sub.b                                                                             V    E.sub.t                         ______________________________________                                        Phase 1                                                                             105    37      224  52    26   89    128  101                           Phase 2                                                                              98    17      256  43    18   123    51  15                            Phase 3                                                                             110    25      190  56    22   74    100  28                            ______________________________________                                         V.sub.s = outlet value,                                                       E.sub.t = standard deviation,                                                 A.sub.b = reduction,                                                          V = value of index.                                                      

The effect of talc on the Mohlmann index was spectacular (ability toseparate solid/liquid), as well as on the consistency of the outletvalues, characterized by very low standard deviations. Not only did talcenable a better mean purification to be obtained, but its quality wasvery consistent.

The effect of talc on the treatment of pollution, both total andsoluble, was undeniable.

EXAMPLE 2

Stabilization of a purification station by activated sludges operatingconventionally, by introducing mica powder.

The phases of the studies and the parameters measured were identical tothose described in example 1. The mica powder was added at a rate of 1.0g/l of water to be treated.

The mica powder used was sold by KAOLINS d'ARVOR (France) under thereference "MICARVOR 20". It consisted of 55% mica, 30% kaolinite and 15%feldspars. 75% of particles had an equivalent spherical diameter lessthan 7.4 μm, 50% less than 4.1 μm and 25% less than 1.9 μm.

    ______________________________________                                        Total COD       Soluble COD                                                   (mg/l)          (mg/l)         M.I.                                           V.sub.s  E.sub.t A.sub.b                                                                              V.sub.s                                                                             E.sub.t                                                                            A.sub.b                                                                             V    E.sub.t                         ______________________________________                                        Phase 1                                                                             105    37      224  52    26   89    128  101                           Phase 2                                                                             100    22      254  48    22   118    86  26                            Phase 3                                                                             110    25      190  56    22   74    100  28                            ______________________________________                                         V.sub.s = outlet value,                                                       E.sub.t = standard deviation,                                                 A.sub.b = reduction,                                                          V = value of index.                                                      

The use of mica powder thus enabled an improvement to be obtained in theremoval of carbonaceous pollution, an improvement in the consistency ofthis removal as well as an improvement in the separation of activatedsludges/purified water.

EXAMPLE 3

Putting a station into conformity which had been out of order due to thepresence of filamentous bacteria, by introducing talc powder.

This example was carried out in four successive phases:

First phase

Duration: 10 days of operation during which the pilot installation wassupplied by the same type of waste waters as those used in examples 1, 2or 3, to which 1.5 g of glucose were added per litre of water to betreated. Glucose is an easily biodegradable pollutant, which encouragesthe development of filamentous bacteria. After the first five days,purification was completely disturbed; the separator was filled withsuspended matter, there was no longer any solid/liquid separation andthe bacteria were expelled from the pilot installation.

Second phase:

Duration: 3 days of operation with 2 g of talc/litre of waste water tobe treated +1.5 g of glucose per litre of water to be treated,introduced continuously into the separator. After these three days,sludge recirculation was once again possible.

Third phase:

Duration: 27 days of operation with 0.15 g of talc/litre of waste watersto be treated +1.5 g of glucose per litre of water, introducedcontinuously into the separator. A reduction in the quantity of talc waspossible since the second phase rapidly enabled the installation to bemanaged normally.

Fourth phase:

Duration: 15 days of operation without talc, with only the mixture ofwaste waters+glucose (1.5 g/l). About a week was needed to remove thetalc completely from the installation.

The parameters measured were identical to those described in example 1.

The talc used was "Luzenac MB30", used and described in example 1.

    ______________________________________                                        Total COD         Soluble COD                                                 (mg/l)            (mg/l)       M.I.                                           V.sub.s    E.sub.t                                                                              A.sub.b V.sub.s                                                                            E.sub.t                                                                            A.sub.b                                                                            V    E.sub.t                         ______________________________________                                        Phase 1 1202   1675   167   186  149  310  499  421                           Phases 2 & 3                                                                           108    77    1795   70   48  505  205   98                           Phase 4 1150   1590   190   173  133  320  510  470                           ______________________________________                                         V.sub.s = outlet value,                                                       E.sub.t = standard deviation,                                                 A.sub.b = reduction,                                                          V = value of index.                                                      

The effect of talc was spectacular. It enabled the value of the totalCOD at the outlet from the installation to be brought down below thestandard of 125 mg/l.

Reduction in the three purifying characteristics of the installation wasvery rapid and very large during phases 2 and 3. However, observation ofbacterial flocs under the optical microscope indicated that the ecologyof the system had not been altered, filamentous bacteria still beingpresent in the system although their harmful effects on solid/liquidseparation were no longer noticed since these bacteria enclosed bywinding round the talc to form dense mixed flocs.

During the fourth phase (discontinuation of talc), the problemsreappeared as the talc was removed (via sludge purges), until theybecame insoluble when all the talc was removed. Consequently, theinstallation could no longer purify waste waters satisfactorily.

EXAMPLE 4

Putting a station into conformity which had been out of order due to thepresence of filamentous bacteria, by introducing mica powder.

The phases of the studies and the parameters measured were identical tothose described in example 1.

The mica powder used was that described in example 3.

    ______________________________________                                        Total COD         Soluble COD                                                 (mg/l)            (mg/l)       M.I.                                           V.sub.s    E.sub.t                                                                              A.sub.b V.sub.s                                                                            E.sub.t                                                                            A.sub.b                                                                            V    E.sub.t                         ______________________________________                                        Phase 1 1202   1675   167   186  149  310  499  421                           Phases 2 & 3                                                                           127    101   1776   84   51  491  250  110                           Phase 4 1197   1605   143   181  138  312  525  478                           ______________________________________                                         V.sub.s = outlet value,                                                       E.sub.t = standard deviation,                                                 A.sub.b = reduction,                                                          V = value of index.                                                      

The use of mica powder thus enabled the installation to operatesatisfactorily, in spite of the presence of filamentous bacteria. Theuse of mica did not alter the ecology of the system.

EXAMPLE 5

Nitrification at constant temperature by adding talc powder.

Example 5 was carried out in the pilot installation already described.In order to study the influence of talc powder additions on thenitrification of ammonium ions, all the pilot installation was kept at15° C. by means of a thermostat (nitrification depends enormously ontemperature, and a difference of a few degrees alters the reactionkinetics enormously).

Since the nitrification reaction also depends on the mass load appliedto the installation, it was studied with and without talc, while varyingthis. This was obtained by altering the hydraulic conditions of theinstallation.

The results obtained are illustrated by the curves 1 and 2 of the singlefigure of drawings, showing changes in the quantity of ammonium ionsremoved from the installation (N-NH₄ ⁺ load eliminated, expressed in kgof N-NH₄ ⁺ /m³ /day) as a function of the ratio of the quantity ofcarbonaceous pollution entering the installation to the quantity ofmicro-organisms present in the activation tank (mass load applied,expressed in kg BOD/kg suspended matter/day).

The talc powder used was that used in example 1, in the same quantities(0.15 g/l of water to be treated).

The curves 1 and 2 show unambiguously that, due to the addition of talc,it is possible to remove large quantities of ammonium ions, even at ahigh or very high load, whereas this removal is very limited orimpossible without talc.

EXAMPLE 6

Putting a public purification station into conformity by addition oftalc powder.

Example 6 was carried out on a purification station with a capacity of1000 inhabitant-equivalent treating between 600 and 700 m³ /day. Thenominal capacity was only 300 m³ /day. This station, situated in Styrie(Austria), was subjected to low temperatures, during experimentation thetemperature being stabilized around 10° C. At this low temperature, thenitrification kinetics were considerably retarded and ammoniumdegradation could not take place.

The purification station had available an effluent pretreatment unit(screen, sand catcher and degreaser). The effluent was then allowed tosettle for about 2 hours before being returned to the activation tank.Separation of bacterial flocs from the purified water was carried out ina secondary separator of a conventional type.

This example was carried out in three successive phases:

First phase:

Duration: 20 days of conventional operation.

Mean temperature: 15° C.

Second phase:

Duration: 35 days of operation with about 0.20 g of talc/1 of wastewaters to be treated. The talc powder, which was difficult to wet, waspre-wetted by water entering the activation tank in an Archimedeanscrew, before being introduced continuously into the activation tank.

Mean temperature: 10° C.

Third phase:

Duration: 45 days of conventional operation, introduction of talc powderbeing discontinued at the start of this phase. About ten days wereneeded to remove talc powder from the station.

Mean temperature: 10° C.

The talc powder used was sold by the Naintsch Mineral Werke Company(Austria) under the reference "Biosorb 30".

It consisted of 55% hydrated magnesium silicate and 45% hydratedmagnesium aluminium silicate. 75% of particles had an equivalentspherical diameter less than 12 μm, 50% less than 6.7 μm and 25% lessthan 3.4 μm.

During the three successive phases, the many pollution indices for thewater discharged were measured daily:

chemical oxygen demand of all the effluent (total COD, in mg/l),

concentration of ammonium ions in the effluent as nitrogen (N-NH₄, inmg/l),

concentration of nitrate ions in the effluent as nitrogen (N-NO₃,inmg/l),

concentration of orthophosphate ions in the effluent as phosphorus(P-oPO₄, in mg/l).

    ______________________________________                                               Total COD                                                                             N--NH.sub.4                                                                              N--NO.sub.3                                                                            P-oPO.sub.4                                       (mg/l)  (mg/l)     (mg/l)   (mg/l)                                     ______________________________________                                        Stage 1  102       28.5       12.5   1.3                                      Stage 2   43        2.5        5.2   0.9                                      Stage 3  112       33.0       14.3   1.5                                      ______________________________________                                    

Mean values at the outlet of the installation of the principal pollutionindicators of the water.

These values may be translated into a reduction due to the talc:

    ______________________________________                                        Total COD     N--NH.sub.4                                                                              N--NO.sub.3                                                                            P-oPO.sub.4                                 ______________________________________                                        Talc    60%       90%        60%    35%                                       effect                                                                        ______________________________________                                    

Reduction in pollution due to talc, compared with periods without talc.

The effects of talc were spectacular and unexpected. The very positiveinfluence of the use of talc powder on the removal of carbonaceouspollution (COD) and ammoniacal pollution (NH₄ ⁺) already observed inpilot installations was found here again. The very positive influence ofthe use of talc powder on the removal of nitrates and phosphates had notbeen anticipated, since all the tests carried out in the laboratory wereon almost perfect pilot installations. In particular, the activationtanks of these pilot installations were completely aerated. In fact,denitrification could not be observed. Now, in a real station, theability to agitate and aerate is not homogeneous over all the volumesand "anoxic" zones exist, that is to say non-aerated zones where only alittle denitrification is conventionally possible. This unexpectedeffect of the additive according to the invention may be explained by"protection" by the talc powder of purifying organisms present in themedium and/or a concentration effect of nitrifying bacteria due to thepresence of talc powder. Indeed, "sludge respiration" measurementscarried out on conventional flocs and mixed flocs indicated that mixedflocs consumed oxygen in the activation tank two to three times morerapidly than conventional flocs.

This indicated that bacterial activity was much greater and couldexplain the increased consumption of phosphorus by bacteria, as well asmore thorough degradation of ammonium, nitrates and carbonaceouspollution.

EXAMPLE 7

Densification of activated sludges by sedimentation in the presence ofvarious additives.

In this example, densification of activated sludges alone and sludgesmixed with calcium carbonate or cationic agents alone, was compared withthat observed after addition of talc, cationized talc or mica accordingto the invention.

The activated sludges came from the public purification station wherethe experiments described in example 6 were carried out. They contained4.1 g of dry matter per litre of activated sludge.

The following mineral materials, cationized or not, were added to alitre of the said activated sludges over 10 minutes:

a) 0.5 g of "Biosorb 30", the talc described and used in example 6,

b) 0.5 g of "Biosorb 30", treated with 0.7% by weight of "Percol"® soldby Allied Colloid (GB), an amine based on epichlorhydrin,

c) 0.0035 g of "Percol"®, that is the quantity added in b),

d) 0.5 g of "Biosorb 30" treated with 0.7% by weight of "Superfloc"®(sold by the American Cyanamid Corporation (New Jersey - U.S.A.)), aquaternary aliphatic polyamine,

e) 0.0035 g of "Superfloc"®, that is the quantity added in d),

f) 0.5 g of "20B", kaolin,

g) 0.5 g of "Micarvor 20", the mica described and used in examples 3 and6,

h) 0.5 g of "Hydrocarb 5", a calcium carbonate sold by the OMYA Company(Switzerland),

i) 0.5 g of "HTM 20", a pyrophyllite sold by MINERACAO MATHEUSS LEMELtda (BRAZIL). This pyrophyllite was more than 95% pure. 75% ofparticles had an equivalent spherical diameter of less than 9 μm, 50%less than 5 μm and 25% less than 2.6 μm.

The mixtures thus obtained, as well as a reference consisting ofactivated sludges alone (Ref.), were introduced into graduated testtubes and the volume of sludges settled were noted after 30 minutes ofsettling.

    __________________________________________________________________________             Ref                                                                              a  b  c  d  e  f  g  h  i                                         __________________________________________________________________________    Volume   790                                                                              470                                                                              410                                                                              750                                                                              380                                                                              750                                                                              670                                                                              630                                                                              750                                                                              460                                       after 30 min                                                                  (ml)                                                                          Reduction in                                                                           -- 40%                                                                              48%                                                                              5% 52%                                                                              5% 15%                                                                              20%                                                                              5% 42%                                       volume compared                                                               with reference                                                                __________________________________________________________________________

These results show unambiguously the important part played by talc,pyrophyllite and mica powders on the sedimentation of activated sludges,as well as the synergy that exists between cationic agents and theseactivated powders, talcs in particular. It will be noted that chemicaladditives employed alone, as well as calcium carbonate, have littleinfluence on the process.

We claim:
 1. An activated sludge waste water treatment process providingimproved purification yields for at least one of carbonaceous,nitrogenous and phosphate pollutants in the waste water, the processcomprising the steps of:a) providing a biomass having purifyingmicroorganisms therein for treating a waste water; b) intermixing ahydrophobic mineral powder selected from the group consisting of talc,pyropyllite and mica with a biomass having purifying microorganismstherein to provide a composite floc containing a microorganism matrixenclosed mineral; c) contacting in an activation chamber the waste waterto be treated with the biomass for a period of time sufficient toproduce an activated sludge having a density greater than water; d)separating the activated sludge from the waste water in a separator; ande) recycling the activated sludge containing the composite flocs withinthe activation chamber to selectively control at least one of theconcentration and residence time of the purifying microorganismstherein.
 2. Process according to claim 1, wherein a talc, pyrophylliteor mica powder is used consisting of particles with a size less than 100μm.
 3. Process according to claim 2, wherein a talc, pyrophyllite ormica powder is used consisting of particles with a size of between 3 and50 μm.
 4. Process according to one of claim 1, wherein between 0.01 and2.0 g of talc, pyrophyllite or mica powder are mixed per litre of wastewater to be treated entering the activation tank.
 5. Process accordingto claim 1, wherein a cationized talc, pyrophyllite or mica powder isused.
 6. The process of claim 1 and wherein:a) the biomass intermixedwith the hydrophobic mineral powder is the separated activated sludge.